Fainting after running: what to know

During a run, leg muscles help return blood to the heart. Stopping suddenly removes that pumping effect while blood vessels may remain widened, so blood pressure can fall. Heat, sweating, illness, inadequate fluids or low blood sugar can add to light-headedness.

The exact timing is useful. A faint that occurs several moments after stopping can have a different pattern from one that happens while running. Palpitations, chest discomfort, unusual breathlessness or a family history of sudden heart problems increase concern, even if recovery is quick.

Stop exercising, lie flat and raise the legs if there is no injury. Move to a cool place, loosen restrictive clothing and have someone stay with you. Once completely alert, drink water slowly and eat if you may have missed a meal. Do not resume training, swim alone or drive until a clinician has advised you.

Call emergency services for fainting during the run, chest pain, severe breathlessness, a sustained racing or irregular heartbeat, seizure-like movements, injury or failure to recover normally. Arrange prompt medical review after any unexplained exercise-related faint, particularly when it repeats or occurs without warning. A clinician may check blood pressure, blood count, glucose and heart rhythm.

Ask whether you need an ECG or exercise restriction while the cause is clarified. Share your pace, weather, fluid intake, meals, medicines, supplements, warning symptoms and whether the blackout happened during running or after stopping.